The present invention relates to a temperature control device for the heat-roll fusing apparatus comprising a heat roll having a heater built therein and a pressure roll disposed in pressure contact with said heat roll. More particularly, the present invention relates to a device adapted to perform warming-up temperature control, that is to say the control of temperature from initiation of power supply till the surface temperature of said heat roll reaches a fusing temperature.
A variety of devices have been proposed heretofore in the art. For example, there is known a device such that in consideration of a temperature increase by overshooting, the heater is switched off at the time when a surface temperature sensor element has detected a preset temperature somewhat below the fusing temperature. Another known device is such that the heater current time is set in accordance with the detection value from a surface temperature element at the time when the power supply is switched on (Japanese Unexamined Patent Application No. 59-34577).
However, the former device in which temperature control is performed on the basis of a result of comparison with a predetermined temperature has the disadvantage that when the roll surface temperature at initiation of power supply is high, i.e. only slightly below the predetermined temperature, the heater current time is so short that the supply of thermal energy to the heat roll is small, with the result that the surface temperature of the heat roll does not rise to the required fusing temperature.
The latter device which performs temperature control according to current time, the thermal energy generated in a given time varies according to changes in source voltage so that when the source voltage is high, overshooting causes damage to the heat roll.
The above trouble occurs more frequently when the set time is longer, that is to say when the initial surface temperature is lower.